Complex Magnetic Order in Topochemically Reduced Rh(I)/Rh(III) LaM0.5Rh0.5O2.25 (M = Co, Ni) Phases

Topochemical reduction of the cation-disordered perovskite oxides LaCo0.5Rh0.5O3 and LaNi0.5Rh0.5O3 with Zr yields the partially anion-vacancy ordered phases LaCo0.5Rh0.5O2.25 and LaNi0.5Rh0.5O2.25, respectively. Neutron diffraction and Hard X-ray photoelectron spectroscopy (HAXPES) measurements reveal that the anion-deficient phases contain Co1+/Ni1+ and a 1:1 mixture of Rh1+ and Rh3+ cations within a disordered array of apex-linked MO4 square-planar and MO5 square-based pyramidal coordination sites. Neutron diffraction data indicate that LaCo0.5Rh0.5O2.25 adopts a complex antiferromagnetic ground state, which is the sum of a C-type ordering (mM5+) of the xy-components of the Co spins and a G-type ordering (mΓ1+) of the z-components of the Co spins. On warming above 75 K, the magnitude of the mΓ1+ component declines, attaining a zero value by 125 K, with the magnitude of the mM5+ component remaining unchanged up to 175 K. This magnetic behavior is rationalized on the basis of the differing d-orbital fillings of the Co1+ cations in MO4 square-planar and MO5 square-based pyramidal coordination sites. LaNi0.5Rh0.5O2.25 shows no sign of long-range magnetic order at 2 K – behavior that can also be explained on the basis of the d-orbital occupation of the Ni1+ centers.

. Observed, calculated and difference plots from the structural refinement of LaCo0.5Rh0.5O3 against SXRD data collected at room temperature. Table S1. Parameters from the structural refinement of LaCo0.5Rh0.5O3 against SXRD data collected at room temperature.
2. Structural Characterisation of LaNi0.5Rh0.5O3. Figure S2. Observed, calculated and difference plots from the structural refinement of LaNi0.5Rh0.5O3 against SXRD data collected at room temperature. Table S2. Parameters from the structural refinement of LaNi0.5Rh0.5O2 against SXRD data collected at room temperature.

HAXPES.
7. Magnetic measurements in the presence of elemental Ni and Co impurities via the 'ferrosubtraction' method. Figure S6. Magnetisation of LaNi0.5Rh0.5O2.25 measured as a function of applied field at 300 K.

Atom
Site  Table S1. Parameters from the structural refinement of LaCo0.5Rh0.5O3 against SXRD data collected at room temperature.

S4
2. Structural Characterisation of LaNi0.5Rh0.5O3. Figure S2. Observed, calculated and difference plots from the structural refinement of LaNi0.5Rh0.5O3 against SXRD data collected at room temperature.  Table S2. Parameters from the structural refinement of LaNi0.5Rh0.5O2 against SXRD data collected at room temperature.

Magnetic measurements in the presence of elemental Ni and Co impurities via the 'ferrosubtraction' method.
Procedure used to measure the magnetization of samples containing elemental nickel: The magnetization of elemental Ni and Co is observed to saturate in applied magnetic fields of more than 2 T. Thus the paramagnetic susceptibility of a bulk sample can be measured in the presence of elemental Ni and Co impurities by measuring the gradient of magnetization-field isotherms in applied fields larger than 2 T. As shown in Figure   S6.
To this end the magnetization of samples was measured in a series of 5 fields between 3 T and 5 T. The magnetization vs. field data were fitted to a linear function, the gradient of which is the paramagnetic susceptibility of the bulk sample and the intercept is the saturated ferromagnetic moment of the sample. Data points with large errors were excluded from fits. All fits had at least 4 data points. This procedure was repeated at 5 K intervals between 5 K and 300 K to measure the temperature dependent susceptibility of samples. Figure S6. Magnetisation of LaNi0.5Rh0.5O2.25 measured as a function of applied field at 300 K. A linear fit to high-field region (H > 25000 Oe) yields a gradient which is the paramagnetic susceptibility of the sample, and an intercept which is the saturated ferromagnetic moment of the sample. S10 8. Structural Characterisation of LaNi0.5Rh0.5O2.25 at 2K. Figure S7. Observed calculated and difference plots from the structural refinement of LaNi0.5Rh0.5O2.25 against NPD data collected at 2 K using the D1B instrument. Black and red and blue tick marks indicated peak positions for the majority phase, a LaNi0.5Rh0.5O3 secondary phase and contributions from the vanadium sample holder, respectively.  Table S6. Parameters from the structural refinement of LaNi0.5Rh0.5O2.25 against NPD data collected at 2 K. S12 9. Magnetic Characterisation of LaCo0.5Rh0.5O2.25. Figure S8. Magnetisation data collected as a function of temperature using the ferrosubtraction technique from LaCo0.5Rh0.5O2.25. Top Panel shows fit of paramagnetic susceptibility to the Curie-Weiss Law in the range 210 < T/K < 300. Middle panel shows paramagnetic susceptibility. Bottom panel shows saturated Ferromagnetic moment. S13 Figure S9. Magnetisation-field data collected from LaCo0.5Rh0.5O2.25 after cooling in an applied field of 50,000 Oe.  Table S7. Nuclear and magnetic structure of LaCo0.5Rh0.5O2.25 determined by refinement against NPD data collected at 2K. The magnetic model is a combination of an mM5 + irreducible representation describing the mx component of the ordered moments (magnetic space group 60.432) and an mΓ1 + irreducible representation describing the mz combination of the ordered moments (magnetic space group 140.541). The cobalt centres in the magnetic model have an occupancy of 0.5.